thanks i got it now it should be like this
2cos(36)sin(18) and multiply by cos18/cos18 as it equals 1
so cos(36)sin(36)/cos(18) and then multiply by 2 and 1/2
so 1/2sin72/cos18 then use co function so sin72= cos18
1/2cos(18)/cos(18) cosines cancel each other so = 1/2